1. Field of the Invention
The present invention broadly relates to acoustic transducers and, more particularly, to a digital audio transducer constructed using microelectromechanical systems (MEMS) technology.
2. Description of the Related Art
Electroacoustic transducers convert sound waves into electrical signals and vice versa. Some commonly known electroacoustic or audio transducers include microphones and loudspeakers, which find numerous applications in all facets of modem electronic communication. For example, a telephone handset includes both, a microphone and a speaker, to enable the user to talk and listen to the calling party. A typical microphone is an electromechanical transducer that converts changes in the air pressure in its vicinity into corresponding changes in an electrical signal at its output. A typical loudspeaker is an electromechanical transducer that converts electrical audio signals at its input into sound waves generated at its output due to changes in the air pressure in the vicinity of the loudspeaker.
Typical relevant art electroacoustic transducers are manufactured serially. In other words, the speakers and microphones are manufactured from different and discrete components involving many assembly steps. For example, the construction of a carbon microphone may require a number of discrete components such as a movable metal diaphragm, carbon granules, a metal case, a base structure, and a dust cover (on the diaphragm). A cone-type moving-coil loudspeaker may require an inductive voice coil, a permanent magnet, a metal and a paper cone assembly, etc. Thus, there is little cost benefit in manufacturing such audio transducers in high volume quantities. In addition, the performance of relevant art electroacoustic transducers is limited by the fluctuations in the performance of the discrete constituent components due to, for example, changes in the ambient temperature, as well as by variations in the assembly process. Variations in the materials and workmanship of discrete constituent components may also affect the performance of the resulting audio transducer.
U.S. Pat. No. 4,555,797 discloses a hybrid loudspeaker system that receives a digital audio signal as an input (as opposed to an analog audio signal typically input to a conventional loudspeaker) and directly generates audible sound therefrom via a voice coil that is subdivided into parts that are connected in series. The voice coil parts are then selectively shorted according to the value of the corresponding bits in the digital audio input word. However, the voice coil may be required to be precisely subdivided for each loudspeaker manufactured. Furthermore, each part of the divided voice coil may need to be precisely positioned as part of the mechanical loudspeaker structure to give an impulse that is accurate to the order of the least significant bit in the digital audio input. The discrete nature of the voice coil exposes it to the consistency, cost and quality problems associated in production and performance of typical loudspeakers as noted above. The voice coils may have to be produced serially with identically manufactured elements so as to assure consistency in performance. Hence, commercial production of instruments incorporating divided voice coils may not be lucrative in view of the complexities involved and the accuracies required as part of coil production and use.
Additionally, solid-state piezoelectric films have been used as ultrasonic transducers. However, ultrasonic frequencies are not audible to a human ear. The air movement near an ultrasonic transducer may not be large enough to generate audible sound.
Accordingly, there exists a need in the relevant art for an electroacoustic transducer which is less expensive to produce and which is smaller in size. It is desirable to construct a solid-state electroacoustic transducer without relying on discrete components, thereby making the performance of the audio transducer uniform and less dependent on external parameters such as, for example, ambient temperature fluctuations. There also exists a need for an acoustic transducer that directly converts a digital audio input into an audible sound wave, thereby facilitating lighter earphones. Furthermore, it is desirable to construct an electroacoustic transducer that allows for the integration of other audio processing circuitry therewith.